Epoxy based anti-corrosion coatings are widely used for the protection of metal and concrete substrates. Due to increasing awareness of environmental protection and severe environmental requirements, low VOC coating system attracted more and more attention from both paint producer and end customers. From technical view of point, one approach to achieve low VOC coating is to prepare high solids coating with reduced solvent addition using low viscosity epoxy.
In the past, solid epoxy resins such as D.E.R.™ 671 (commercial product of the Dow Chemical Company) are widely used in anti-corrosion coatings. However, D.E.R.™ 671 is not suitable for the low VOC high solids epoxy coatings because of its solid state. Large amount of solvent is demanded to dissolve and dilute them which hinder the epoxy resin from low VOC high solids applications. Usually, D.E.R.™ 671 is supplied with 25% xylene and the resin solution commercial grade name is D.E.R.™ 671-X75.
The coating based on liquid epoxy such as D.E.R.™ 331 (commercial product of the Dow Chemical Company) requires less solvent, however, is quite brittle. This is a critical issue for an anti-corrosion coating which requires high flexibility and good adhesion on substrates.
Reactive diluents can significantly reduce the viscosity of the system but at the same time compromise the reactivity or functionality of the resin part. This brings long drying time or poor chemical resistance to the coating. Meanwhile, reactive diluents are always more expensive than epoxy resins.
Fatty acid modified epoxy resins are one kind of popular modified epoxy in low VOC high solids coatings. However, the ester group generated from the reaction between epoxy and acid has the risk to hydrolyse, since ester group is inclined to hydrolyze.
It is therefore, still desired in the art an epoxy resin giving both lower viscosity and higher flexibility so that it is very suitable for the low VOC high solids coating application.